Methods for the removal of glass are known from the state of the art. Different removal methods can be differentiated, e.g. mechanical, chemical and removal by heating.
A method for the removal of glass is described in JP58079835. The substrate is positioned in the inside of a tube and the space between substrate and tube is filled with an etching gas (consisting of helium and sulphur hexafluoride). From the outside of the tube energy is applied by means of an oxygen/hydrogen burner and the substrate with the etching gas is heated, so that material of the substrate is removed.
A similar method is described in JP62059545 whereby the aim of this invention is not to remove material but to remove contaminations. Therefore an etching gas is applied between the core rod and cladding tube to remove the contaminated outer layers of the core rod and the inner layers of the cladding tube. Subsequently the cladding is collapsed onto the core rod to yield a preform.
JP64009831 describes a method for the overcladding of substrates with a plasma burner, wherein the plasma burner is used in a preparation step to remove contaminations from the surface and therefore reduce the OH-content.
JP4209729 describes a method which removes material by a plasma burner and sets the amount of etching gas according to a diameter measuring device to yield a target diameter. The method is only used in one direction and uses a plasma burner with argon/oxygen mixture.
JP2010013352 describes a method for the removal of glass material from a preform surface prior to further processing. A layer with less than 0.3 mm is removed to minimize material loss.
US2008028799 describes a multi-step process for the removal of glass material. In a first step after the inside deposition by a MCVD (modified chemical vapor deposition) process the tube is etched on the inside to remove contaminations (especially OH). Afterwards the tube is collapsed to a rod. The contaminations of the surface with OH applied during deposition and collapsing are removed in a second process step either with a plasma flame or by wet chemical etching.
WO03052173 describes a method for a two-step etching of a glass tube. By a MCVD process layers are deposited on the inside of the glass tube which is partly collapsed afterwards. The resulting capillary is used for etching by applying an etching gas inside and heat produced by an oxygen/hydrogen burner on the outside. Within a first run the removal rate is maximized to optimize the process economy. The resulting contaminations with etching medium are removed in an additional etching run with a low concentration of etching medium to remove only a small amount of material.
JP2004010368 describes another method for the removal of contaminated glass layers.
In U.S. Pat. No. 7,946,134 a method for the production of preforms with large diameters is disclosed. This can be achieved by producing a very large core inside a substrate tube by a MCVD process. This bears the problem, that the substrate tube will become the inner cladding layers in the final preform. This is disadvantageous because usually an oxygen/hydrogen burner is used during MCVD, which causes OH-contaminations of the substrate tube. The proposed solution is to remove a large part or even the complete substrate tube. Now a cladding can be applied to the core which is not produced with oxygen/hydrogen flame. This improves the attenuation of the fibers drawn by such preforms.
EP1475358 describes a method for the removal of a substrate tube or the preform to reduce its ovality. This is achieved by controlled set-up of rotation and translation of the substrate and glass removal at specific positions. It is either possible to treat the substrate tube prior to deposition to carry out the MCVD process with a small formation of core ovality, or the cladding of preform with oval cores is treated in such a way, that the complete preform becomes oval but the layer thickness of the cladding material becomes equal at every position. During fiber drawing of the oval preform the drawing temperature is set to obtain circular fibers because of the surface tension.
In the methods known from the state of the art often only thin layers are removed to remove contaminations.
Furthermore in many cases the etching is performed inside a tube only in one direction. To obtain a maximized removal rate the etching should be carried out in both directions, which causes some problems.
By using a bidirectional removal process according to the state of the art especially the following disadvantages occur.
By the bi-directionality of the removal process in the area of the removal point an uneven temperature distribution occurs. This results in an inhomogeneous temperature distribution over the length of the preform (also called substrate). This inhomogeneous temperature distribution results in an inhomogeneous removal rate and a deviation of the preform from the target shape.
Furthermore the diameter of the preform cannot be measured accurately in the burner flame. This results in the problem that there is no possibility to control the process according to a target diameter. The diameter measurement is hindered especially by the radiation of the heating instrument, which emits electromagnetic radiation. Therefore the diameter measurement can only be carried out before or after the position where the burner heats the substrate.
To reduce the prior discussed uneven temperature distribution very long handling rods can be applied to the substrate. These handling rods are melted at the ends of the substrate and put in a glass working lathe to set the substrate/preform to rotate. Thereby the removal point can be displaced from the preform to the handling rods. This yields a removal of material of the handling rods which requires periodic renewal of the handling rods. Furthermore the etching time is enhanced and the economy of the process reduced.
Especially in the case in which the handling rod and the preform are made of different glass materials, a diameter neck at the connection might be formed. This is mainly based on differing viscosity of the different glass materials. When the preform is heated this might end in a local displacement at the connection point. This is compensated when the preform is cooled down but there might occur some material displacement, which reduces the quality of the resulting glass fiber.
Especially in the case in which a number of removal runs are necessary to yield the target shape of the preform, different amounts and therefore layer thicknesses have to be removed from the preform. The amount of glass material which can minimally be removed per run depends strongly on the diameter of the preform. It should be avoided to remove less than 0.3 mm for a cylindrical target with diameters of less than 30 mm, because this would result in relative translation speeds between burner and preform, which are so high, that the amount of glass removed per unit time becomes uneconomically small.